Recently, a circuit substrate comprising a metal foil having formed thereon an insulating layer comprising a polyimide resin has been used as a thin-film multilayer circuit substrate for the purposes of high-density packaging of semiconductors and a high-speed signal treatment. However, in such a conventional circuit substrate, since the polyimide resin has a large hygroscopic property, the circuit substrate is liable to cause a dimensional change and warping by the change of the humidity in the environmental atmosphere.
On the other hand, in computers and storage devices which are peripheral equipments for computers, in addition to the improvement of the capacity, small-sizing and a low cost have been required and with such requirements for a background, in particular, the technique of hard disk drives has been greatly advanced. Also, in the magnetic head, recently the developments of a thin-film magnetic head (TFH) wherein the coil portion is made of a thin film and further a thin film-magnetic resistance composite head (MR) which serves reading and writing and has a greatly large storage capacity have been hurried to a conventional metal in gap (MIG).
However, by a conventional technique of constituting desired wiring by laying a conductive wire on a suspension substrate, the conductive wire gives influences on the elastic modulus of the suspension to cause the fluctuation of the pushing force of the suspension and as the case may be, by the contact with the magnetic disk, the durability of the magnetic disk device is sometimes lowered.
Therefore, recently, a suspension formed by directly forming an electric circuit on a suspension substrate for mounting a magnetic head has been practically used. However, according to the conventional circuit substrate using a polyimide resin as an insulating layer, in a suspension finally obtained, due to that the polyimide resin has a coefficient of thermal linear expansion larger than that of a metal substrate, a dimensional change and warping occur on the circuit substrate, and the inferior performance sometimes occurs.
Further, the conventional polyimide resins are poor in adhesion of polyimide resins each other. For example, in the production of a suspension, there is a problem that it is difficult to form a cover layer.